The present invention relates generally to nitride based semiconductor structures, and more particularly to a method of fabricating a nitride based resonant cavity semiconductor structure by laser-assisted epitaxial lift-off to move the semiconductor structure from a first substrate to a second substrate to allow fabrication of distributed Bragg reflectors (DBRs) on both sides of the nitride based resonant cavity semiconductor structure.
A planar multi-layered semiconductor structure can have one or more active semiconductor layers bounded at opposite sides with semiconductor layers that form distributed Bragg reflectors. The distributed Bragg reflectors at opposite sides of the active semiconductor layer are formed from alternating high refractive index and low refractive index quarter-wavelength thick semiconductor or dielectric layers that function as mirrors. The multiple semiconductor layers including the active semiconductor layer, between the opposing distributed Bragg reflectors, form a resonant cavity for light emission or light absorption within the semiconductor structure. The active semiconductor layers within the resonant cavity will either emit light for a light emitting diode (LED) or vertical cavity surface emitting laser (VCSEL) or absorb light for a photodetector (PD).
The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. These layers with excess electrons are said to be n-type, i.e. negative. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. These layers with excess holes are said to be p-type, i.e. positive.
A forward biased electrical potential is applied through electrodes between the aside and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to xe2x80x9cinjectxe2x80x9d them into the active layers, where electrons recombine with holes to produce light. A light emitting diode will emit light from the resonant, cavity through one of the DBRs through the upper or lower surface of the semiconductor structure. For a laser, optical feedback provided by the opposing DBRs allows resonance of some of the emitted light within the resonant cavity to produce amplified stimulated emission of coherent xe2x80x9clasingxe2x80x9d through one of the DBRs through either the upper surface or the lower surface of the semiconductor laser structure.
For a photodetector, a reverse biased electrical potential is applied through the electrodes between the p-side and the n-side of the layered structure. A photodetector will absorb light in the active layer of the resonant cavity, thereby driving electron/hole pairs from the active layer to be collected to form a photocurrent.
Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V semiconductors, comprise elements selected from group III, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as aluminum gallium indium nitride (AlGaInN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum.
These materials are particularly suited for use in short-wavelength VCSELs or LEDs for several important reasons. Specifically, the InGaAlN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under the high electric current and the intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
Semiconductor resonant cavity structures comprising nitride semiconductor layers grown on a sapphire substrate will emit or absorb light in the near ultra-violet to visible spectrum within a range including 280 nm, to 650 nm, allowing better efficiency and narrower line widths for LEDs and photodetector.
The shorter wavelength blue of nitride based semiconductor VCSELs and LEDs provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) VCSELs and LEDs for high-resolution or high-speed laser printing operations and high density optical storage. In addition, blue light emitting devices can potentially be combined with existing red and green lasers or LEDs to create projection displays and color film printers.
In many applications, the conventional substrate material for semiconductor structures would be silicon or gallium arsenide. However, the GaN crystal structure, combined with the high GaN growth temperatures, make deposition of high-quality nitride semiconductor material directly onto semiconductor substrates such as Si or GaAs very difficult.
Nitride based semiconductor structures currently require heteroepitaxial growth of GaN thin layers onto dissimilar substrates such as sapphire or silicon carbide.
The most commonly used growth substrate, sapphire, still imposes constraints on the GaN layer quality due to the lattice and thermal-expansion coefficient mismatch between the GaN and the sapphire. The disparate properties of these two materials result in a high density of extended defects, such as dislocations and stacking faults, at the GaN thin layer/sapphire substrate interface.
Many substrate separation techniques are available including wet-chemical etching, chemical-mechanical polishing or laser-assisted lift-off. Wet-chemical etching and chemical-mechanical polishing are inherently slow processes that require high selectivity in materials in order to remove the original growth substrate. Laser assisted lift-off processes have several advantages over the chemically assisted methods for the GaN thin film/sapphire substrate system. The laser processing is optically selective, possesses spatial control and is a relatively fast lift-off technique.
In order for the substrate separation technique to be successfully implemented, the technique itself must not degrade the quality of the GaN layer being processed. The laser process introduces a thermoelastic stress to the GaN layer, due to the rapid heating and cooling during the pulsed irradiation, that may fracture the GaN layer. Thin film fracture may arise from microcracks within the biaxially stressed GaN or from a thermal shock initiating microcrack propagation through the GaN layer.
An inherent problem when depositing thick GaN layers heteroepitixally onto sapphire or GaAs is the intrinsic stress, compressive for sapphire and tensile for GaAs, regardless of the substrate separation technique, due to the thermal coefficient mismatch between the GaN film and the substrates.
The success of the growth substrate removal to create a GaN substrate is dictated, in part, by the quality of the as-grown GaN layer. Due to complications related to heteroepitaxy, thick GaN layers, like those needed for a substrate, generally possess microcracks that can propagate and multiply during the laser lift-off process. The combination of the intrinsic residual stress and the thermoelastic. stress of the laser processing gives rise to crack propagation across the entire GaN wafer area. The crack propagation would lead to uncontrolled catastrophic mechanical failure of the GaN or, at least, ill-defined low-quality GaN substrates.
Another problem specific to fabricating GaN VCSELs is the difficulty in growing the highly reflective AlGaN-based distributed Bragg reflectors (DBRs) needed for stimulated emission of coherent light of VCSELs, where the minimum aluminum content for the AlGaN layers in the DBRs is limited by self absorption of the light and the maximum aluminum content is limited by lattice matching constraints.
Similar problems plague the long wavelength indium phosphide VCSELs but the problem in phosphide based laser structures can be solved by etching a hole through the substrate and evaporating dielectric materials to form the DBR. Unfortunately, the usual substrate for nitride based structures, i.e. sapphire, is difficult to dry or wet etch, so that this back-etch procedure is not available to the fabrication of the nitride based laser structure.
It is an object of the present invention to provide distributed Bragg reflectors (DBRs) on both sides of the nitride based resonant cavity semiconductor structure.
According to the present invention, a nitride based resonant cavity semiconductor structure with a first distributed Bragg reflector is fabricated on a sapphire substrate, a second substrate is bonded to the first distributed Bragg reflector, the sapphire substrate is removed by laser-assisted epitaxial lift-off, and a second Bragg reflector is fabricated on the VCSEL structure opposite the first distributed Bragg reflector. The nitride based resonant cavity semiconductor structure can be a VCSEL, LED or photodetector, or a combination of said devices.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.